Adaptive filters operate on a signal supplied thereto in accordance with a prescribed criterion to generate a desired output signal. Typically, the filters generate a transfer function (an impulse response characteristic) in accordance with an algorithm which includes updating of the transfer function characteristic in response to an error signal. In this way the filter characteristic is optimized to yield the desired result.
It has been found advantageous to normalize the update gain of the adaptive filter. The normalization serves to make the performance of the filter insensitive to variations in received signal power. In one prior arrangement an average of the squares of input signal sample magnitudes is used to normalize the gain, as described in an article by Mr. D. L. Duttweiler entitled "A Twelve-Channel Digital Echo Canceler", IEEE Transactions on Communications, Vol. COM-26, No. 5, May 1978, pp. 647-653. Another gain normalization arrangement employing a sum of the squares power estimate is disclosed in U.S. Pat. No. 3,922,505 issued Nov. 25, 1975.
Although these prior arrangements perform satisfactorily in some applications, poor performance or possibly instability results when the received signal includes transient signals, rapidly pulsating signals or the like. In telephone applications, instability may result in echo canceler filters for busy telephone signaling, multifrequency tones, other pulsating signals, data sets coming and going, or the like. The instability results because the average power estimate used in prior arrangements is relatively small for the transient signals thereby generating an update gain which is too large. Even if the filter does not become unstable, at best very poor convergence results, i.e., the characteristic converged to does not closely represent the desired characteristic. These results are undesirable.